Conventionally, theft of vehicles has been prevented by using an ignition key switch adapted for a mechanical cylinder lock. However, since a mechanical key can be duplicated without much difficulty, such a precaution has not always been successful. Japanese Patent Laid-Open Publication No. 57-90237 discloses a vehicular anti-theft system in which starting of the engine is made possible only when a signal containing a code unique to the vehicle is transmitted from a portable transmitter to a receiver mounted on the vehicle after an ignition switch has been closed by inserting a mechanical key into the ignition key switch, and the receiver permits the starting of the engine according to the result of verification of the code. According to this proposal, a duplicated mechanical key cannot start the engine and the theft of the vehicle is positively prevented. However, this patent publication merely suggested the use of a transmitter as means of a double security measure without any specific disclosure as to the physical structure of the system. On the other hand, U.S. Pat. application No. 121,321 assigned to one of the assignees of the present application discloses a similar device which combines an electronic ignition key system and a remote controller for a door lock into a single body.
However, these previously proposed systems have the shortcoming that a series of complicated steps, including the closure of the key switch for starting the engine, transmitting of the signal from the transmitter to the receiver on the vehicle, and the actual starting of the engine, are required. Thus, a certain time delay is inevitable between the time the key switch is turned on and the time the engine is actually started.
Further, in such systems, the transmitter and the receiver typically consist of a light emitting diode and a photodiode, respectively, and an optical pulse code signal is directed from the transmitter to the receiver which is provided, for instance, adjacent to the door handle of the vehicle or the key cylinder of the steering lock. When such a transmitter is used for starting the engine, the duration of the code signal is desired to be as short as possible in order to eliminate any time lag before the engine is actually started. However, when the transmitter is used to unlock or lock the door, the duration of the code signal is desired to be sufficiently long because the user holding the transmitter must aim it at the receiver and may have a substantial difficulty in doing so if the duration of the signal is too short. When starting the engine, however, the transmitter which is accommodated in the handle of the engine key is automatically aligned with the receiver as the ignition key is inserted into the key hole of the ignition switch.
Additionally, if the transmitter is to be used for both as an electronic ignition key system and a remote controller for a door lock, it must be provided with an internal battery and the battery must be electrically charged to a sufficient level at all time for its satisfactory operation. This is particularly important when the transmitter is to be accommodated in the small space of the key handle. By electrically charging the internal battery while the key is inserted in the key hole of the ignition system by establishing an electric contact therebetween, this problem can be solved. But, if the transmission of the signal is continuously performed between the transmitter and the receiver throughout the time in which the power source of the portable unit is electrically charged, the electrical charging of the power source of the transmitter may become insufficient.
According to conventional mechanical key ignition systems, if the owner of the vehicle either has lost or damaged the mechanical key, he can readily duplicate the key without any substantial inconvenience. However, according to the electronic anti-theft system, if he has lost the transmitter or the transmitter has failed to be able to function properly, a considerable inconvenience is caused to him because it takes a considerable time period before another transmitter capable of transmitting the unique code signal becomes available to him, and he cannot operate the vehicle during this time period.